A LCD (liquid crystal display) is a passive light emitting device, in which a BLU (Backlight Unit) is required to provide a light source for enabling the LCD to display an image. Currently, the popular backlights for LCD comprise CCFL (cold cathode fluorescent lamp) and LED (light emitting diode). LED has been increasingly accepted as a backlight due to its advantages of high brightness, high color purity, long service life, high reliability, and free from mercury pollution.
A LED backlight can be classified into a direct type and a side type. In the side type, the light source is placed at a side of the display panel. In the direct type, the light source is arranged at a lower surface (i.e., the non-display surface) of the display panel. Particularly, a construction for a LED backlight of the direct type is shown in FIG. 1.
However, as for the existing LED backlight of the direct type, LED strips 110 act as light sources for emitting light ray. When light ray perceived by human eyes 111 is perpendicular to a display panel 112, the perpendicular light ray has the largest brightness at its intersection point with the display panel 112, because most of the light ray emitted by the light source 110 impinges on a central area of the display panel 112. When light ray perceived by human eyes 111 forms an angle α (0°≦α≦90°) with the normal of the display panel 112, the light ray in the direction of α=0 exhibits the largest intensity. The brightness of light ray at its intersection point with the display panel 112 decreases with the increase of α. In this manner, the brightness across the display panel 112 is not uniform. For instance, when human eyes 111 observe the display panel 112 at a small distance, the brightness at point C is relatively high, while the brightness at point A and point B is relatively low. Or, as shown in FIG. 2, when a test equipment 113 performs a display test on the display panel 112, a lens of the test equipment 113 lie in the perpendicular direction with respect to point C′ on the display panel 112, while forms an angle with point A′ and point B′. Thus, the brightness at point C′ is relatively high, while the brightness at other points is relatively low. In this manner, the average brightness across the display panel 112 will be reduced, thus decreasing the energy efficiency of the display test. In addition, in the practical observation, this will result in a non-uniform brightness of the display panel perceived by the user, thus affecting the display quality.